Bipolar junction transistors are constructed from doped semiconductor material and are often used in switching and amplifying applications.
A problem with bipolar junction transistors is that the high base recombination current is a main contributor to degradation of low current gain. Under typical operation of e.g. an NPN transistor, electrons injected from the emitter pass through the base. These minority carriers are subject to recombination giving rise to an additional hole current from the base. This parasitic current mechanism degrades the common emitter gain of the bipolar transistor. This effect is particularly pronounced in the low current region where the mechanism is a sizable contribution to the overall base current.
In order to address the above problem, U.S. Pat. No. 5,336,625 describes forming a protective structure from a gate material which protects a base active region of bipolar transistors from particle contamination at the low temperature oxide deposition step. This prevents plasma damage at the ashing and resist strip steps following the NMOS transistor source/drain implantation step. The protective structure is subsequently removed to expose the active area of the bipolar transistor prior to the formation the bipolar transistor's base region.
U.S. Pat. No. 6,828,650 describes utilizing a trench positioned between the emitter and base of a transistor to reduce the current gain variation by limiting recombination. The pullback trench structure takes advantage of the lateral component of the emitter current allowing it to contribute to the current gain of the device.